Mystery Engine/Exhaust Problem in '95 Montero LS (HELP!)

[Rick1013]

I've got a '95 Montero LS 3.0L with about 200,000 miles on it. It's got decent (equal-ish) compression in all cylinders. I bought it pretty cheap ($1,000) and had a mechanic put more than that amount of money into replacing and repairing things.

After working on it for a while, he's got most of the "bugs" ironed out except for one.

When the truck is running and up to temp, the engine starts "popping", making a backfiring sound about every 8-10 seconds at idle. My mechanic thinks that the engine is getting too much fuel and that is causing it to "pop". He hasn't been able troubleshoot the reason why. He pulled the new catalytic converter out of concern that the "popping" or excess fuel will damage the cat, but he can't figure out how to make the engine run properly.

The Montero has got brand new spark plugs and wires. A basic tune-up didn't fix anything. Anyone have any troubleshooting ideas I can pass on to my mechanic?

I really appreciate your insight and the advice posted in this forum. I can't wait to actually DRIVE this truck, but it's got to be running right before it can pass WA emissions and get licensed!

[97discosd]

Bad spark plug and or cracked plug. Did you replace the plug wires also?

[Rick1013]

Brand new platinum plugs, brand new wires...

Any other ideas?

[TRegasaurus]

Check EGR and Tube for blockage, the timing could be off, also a stretched timing chain or belt, if your model year has a Mass Airflow Sensor that may be suspect as well.

[Rick1013]

Thanks, guys. Keep the ideas coming, please. I would love to give my mechanic (a great guy who has really busted his butt working on this Montero) all the advice you can offer.

[Cat Fuzz]

Rick's mechanic here. Let me try to elaborate on what's going on. The plugs are new double platinums and the wires are also new. I checked and re-checked the timing belt. I can't count how many times. I even tried a new belt and it still ran the same. One of the issues that is cropping up that is baffling me that when using my inductive timing light to test each cylinder for spark, I can't see any spark on #1 or #3 with the timing light. However, when I pull the wire on #1 and insert a spark plug, while leaving one in the cylinder, I can see the plug sparking. I haven't checked #3 since it requires removing the throttle body to pull the wire. #1 and #3 are fired by different coils. I swapped the coils around and nothing changes. I even replaced those two plug wires under warranty and nothing changed. I pulled the plugs and they seemed gassy so that tells me it's getting too much fuel for some odd reason. The old cat was already hollowed out so I put it back in to act as a test pipe so I won't trash the new cat. When idling it only runs a smidge rough. When accelerating off idle, it's sluggish, just like a bad cylinder would act. This is why I figured a tune-up would fix it but it hasn't. I also swapped the fuel injectors. There was an extra set in the back of the truck that looked different than what was in there. I researched and it seems as though the extra set were the correct ones. Nothing changed.

Here is a video with sound of the popping noise.

http://www.youtube.com/watch?v=d5VWq...&feature=feedu

[scrubber3]

Have you tested the fuel pressure regulator? It's very possible that could be the culprit. Also, check the fuel pump as well. A faulty fuel pressure regulator can cause the fuel pump to get weak, especially with 200,000 miles on it. You should probably have them both changed if you find one is bad. This is what it sounds like to me, but without putting hands on it, it's hard to tell. Test them out and see what you find. The fuel pressure should be 47 to 50 psi. If you hook a tester directly to the hose, It should read around 94 to 100psi.

Hope this helps

Oh, the popping noise MAY just be something stuck in the muffler like a piece of the old catalytic converter.

[Cat Fuzz]

So weird. When I have #1 and #3 plug wires plugged into the coil normally, no spark. When I pull them out and hold them just inside where the conductor goes in, I begin to get spark. What the heck is going on here? I plug them back in, spark goes away.

I pulled it down to the timing belt again and it appeared one cam was off a tiny bit. I retensioned the belt and it lines up now. I'm wondering if a new hydraulic tensioner is needed. I keep getting different info on how much of the rod is supposed to protrude so I'm not sure if I'm getting it right. Anyone know the spec? If I can't get it to spec, do I need a new tensioner? As it sits, I have about .08 inches protruding.

[scrubber3]

ALWAYS put a new tensioner in when you change timing belts. Who knows when the last time it was replaced.

[Cat Fuzz]

But what about the weird coil thing? And the rod protrusion spec?